A sheet processing apparatus including a controller configured to move a carriage from an initial position to such a position that an optical sensor traverses one of two side edges of a sheet in a scanning direction, in a direction along the scanning direction, in response to the sheet reaching a position opposed to the optical sensor, adjust a light emitting quantity of the optical sensor while the carriage is being moved, within a period from when the carriage begins to move from the initial position to when the optical sensor traverses the one side edge, calculate a position of the one side edge traversed by the optical sensor after the adjustment, based on detection values, corresponding to the light receiving quantity, output from the optical sensor after the adjustment, and identify edge positions of the two side edges based on the calculated position of the one side edge.